1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus configured to make a sheet bundle by binding a plurality of sheets.
2. Description of the Related Art
Lately, a configuration of a printing system such as an image forming apparatus, e.g. a copier, a printer, a facsimile, or the like, including a sheet processing apparatus that implements various processes such as binding, punching, sorting, or the like on a sheet on which an image has been formed is widely used. While metallic staples are often used in general in binding sheets in such a sheet processing apparatus, a part of the staple projects above a surface of a sheet bundle when such metallic staple is used.
However, when stapled sheet bundles are stacked one after another on a stacking tray, there is a case when a staple of a sheet bundle to be discharged next is caught by a staple of the sheet bundle already stacked on the stacking tray, thus disturbing alignment of the sheet bundles on the stacking tray. Therefore, there has been proposed such a sheet processing apparatus configured to change stapling position of sheet bundles per every predetermined number of bundles as disclosed in Japanese Patent Application Laid-open No. H9-58924.
However, even though the sheet processing apparatus is to produce products of one and same job, the sheet processing apparatus ends up producing different products because the stapling position of the sheet bundle is differentiated when the sheet processing apparatus changes the stapling position per predetermined number of bundles. Still further, it is necessary to shift a stapling unit largely at least more than a widthwise size of a staple as a shift length of the stapling position to prevent the staples from interfering with each other in view of variations of obliqueness in discharging sheet bundles and a widthwise shift of the sheet bundles in dropping to the stacking tray. If the stapling position of the sheet bundles is shifted largely, the sheets might be turned unevenly, thus damaging the sheets and making the sheet unimpressive.
Still further, although it is possible to solve such a problem that the staples are caught from each other by changing the stapling position, there is a case when a projecting portion of the staple of the already stacked sheet bundle catch an end of a sheet bundle to be discharged next when the stapled sheet bundles are stacked one after another on the stacking tray. In this case, ends of stacked sheet bundles are not justified and tilt on the stacking tray. Thus, the alignment of the sheet bundles is disturbed.